1. Field of the Invention
The present invention relates to an electronic device, and more particularly to an oscillator having time-variant frequency deviation and related power supply.
2. Description of the Prior Art
A power supply that converts an alternating current (AC) mains voltage to a direct current (DC) voltage can maintain an output voltage, current or power within a regulated range for efficient and safe operation of an electronic device. Compared with a traditional linear power supply, a switching power supply utilizing a pulse width modulation (PWM) controller has been used widely due to the high efficiency and generally includes an input stage, a power factor correction (PFC) stage, a power stage and a feedback stage. The input stage can convert an AC mains voltage to a DC voltage; the power factor correction stage can adjust a power factor of the power supply to conform to various product specifications; the power stage includes the driving components and circuits such as a power switch and the pulse width modulation device, etc. The PWM controller cooperating with the oscillator controls a switching frequency of the power switch according to the load condition outputted from feedback stage to achieve stable voltage or stable current. However, in order to achieve the goal, the switching frequency of the power switch is usually higher than the frequency of the AC mains voltage, thereby causing partial high frequency signals to be injected back into the AC mains input and become part of the AC mains signal. In addition, such high frequency signal operation radiates the electromagnetic wave that causes electromagnetic interference (EMI) and affects the inner and outer electronic components of the power supply.
In order to solve the abovementioned electromagnetic interference problem, some documents as below discloses ways for producing the time-variant frequency deviation during the pulse width modulation.
U.S. Pat. No. 6,107,851 discloses an oscillator with pulse width modulation for producing frequency jitter by means of the variable current generated by the outer transistor (such as the transistor 500 shown in FIG. 3 of U.S. Pat. No. 6,107,851) of the oscillator. And U.S. Pat. No. 6,229,366 also discloses the similar method to generate frequency jitter.
U.S. Pat. No. 6,249,876 discloses a primary oscillator, a counter and a current source set for producing frequency jitter, as shown in FIG. 1 of U.S. Pat. No. 6,249,876. When the counter is controlled by the oscillator, each output can represent a specific time interval, and the current source set controls the primary oscillator to change the oscillatory frequency according to the specific time interval to generate frequency jitter.
U.S. Pat. No. 7,026,851 discloses a pulse width modulation controller with frequency jitter. In the pulse width modulation controller 2 of FIG. 2 of U.S. Pat. No. 7,026,851, the modulator 300 generates the jitter current Iscan and Iadj according to the pulse signal PLS and the reference current IREF. The reference current IREF generated by the oscillator 200 is used for determining a switching frequency affected by the modulation of the jitter current ISCAN.
The abovementioned 4 US patents all use the method of inputting an outer variable current into the oscillator to generate frequency jitter.
In addition, U.S. Pat. No. 7,203,079 discloses a switching controller having frequency hopping for reducing the EMI of a power supply. The power supply of this patent includes a pattern generator, an oscillator, and a programmable capacitor (e.g. the programmable capacitor 290 of U.S. Pat. No. 7,203,079). The pattern generator generates a digital pattern code in response to a clock signal. The oscillator generates an oscillation signal for determining a switching frequency of a switching signal. The programmable capacitor coupled to the oscillator modulates the switching frequency in response to the variation of the digital pattern code.
U.S. Pat. No. 7,061,780 discloses a controller which includes a pattern generator for generating a digital pattern code to control a programmable capacitor that is connected to an oscillator (as the programmable capacitor 910 shown on FIG. 3 in U.S. Pat. No. 7,061,780). In this way, frequency hopping can be achieved to reduce the EMI effect.
The abovementioned 2 US patents both utilize the variable capacitor outside the oscillator to input a variable voltage into the oscillator to generate frequency jitter.